The bulk and shear moduli of dense polycrystalline oxygen-bearing c-Zr3N4 were determined to be B0 = 217(20) GPa and G0 = 163(9) GPa, respectively, using laser ultrasonic technique combined with a numerical analysis of the sample porosity. While the obtained B0 is in excellent agreement with the earlier high-pressure compression measurements, the G0 value is 70% higher than the previous estimate. Since both G0 and hardness of the dense c-Zr3N4 exceed those of γ-Si3N4, c-Zr3N4 vies for the rank of the third hardest material after diamond and cubic BN. Our results also support the suggestion that shear modulus is a robust predictor of hardness.